Memory programming to reduce thermal disturb

ABSTRACT

A resistive memory array is programmed such that particular adjacent pairs of memory cells along a bit line having a back-to-back relationship are programmed together. The memory cells having the back-to-back relationship share a continuous chalcogenide material and a SiN material.

TECHNICAL FIELD

The present embodiments relate generally to memory and a particular embodiment relates to memory programming to reduce thermal disturb.

BACKGROUND

Non-volatile memory is presently designed into a large number of electronic devices that require relatively permanent storage of data even after power is removed. Common uses for non-volatile memory include personal computers, solid state drives, digital cameras, and cellular telephones. For example, program code and system data such as a basic input/output system (BIOS) are typically stored in non-volatile memory for use in personal computer systems.

Typical types of non-volatile memory include magnetic disk drives, optical disk drives, flash memory, and phase change memory (PCM). Flash memory has typically been the most common type of non-volatile memory used in electronic devices that are small and battery powered. However, flash memory is relatively slow to program that can result in a potential data bottleneck when implemented in a high speed system.

PCM is a resistive memory technology that can provide non-volatile storage but has the potential of relatively faster operation compared to flash memory. PCM, as the name implies, uses the change in resistance of a material when it changes phase in order to store data in a non-volatile manner. For example, an alloy of different elements might change from a crystalline phase having a low resistance to an amorphous phase having a high resistance. If the material could exhibit multiple distinctly different resistances, each different resistance can then be assigned a respective data value (e.g., 00, 01, 10, 11).

The phase change in PCM is brought about by heating the phase change material of each memory cell when it is addressed. This can be accomplished by a heater for each memory cell. When the heater is enabled by a current, it heats a chalcogenide alloy (e.g., germanium, antimony and tellurium (GeSbTe) or GST). When GST is heated to a relatively high temperature (e.g., over 600° C.), its chalcogenide crystallinity is lost. The GST cools into an amorphous glass-like state having a high electrical resistance. By heating the chalcogenide alloy to a temperature above its crystallization point but below the melting point it will transform back into a crystalline state having a lower electrical resistance.

FIG. 1 illustrates a schematic diagram of a portion of a typical prior art PCM array 100. The PCM array 100 includes a number of memory cells 101, each including a select device 110 coupled to a resistive storage element 111. The select devices 110 can include, for example, field effect transistors (FETs), such as MOSFETs, or bipolar junction transistors (BJTs), or diodes.

Referring to FIG. 1, the select device 110 is shown as a three terminal FET where the gate of each select device 110 is coupled to one of a number of access lines WL0-WLn (e.g., word lines). Each word line WL0-WLn is coupled in such a fashion to its respective row of memory cells. A second terminal of each FET is coupled to its respective resistive storage element 111. A third terminal of each FET is coupled to a circuit common reference. Each resistive storage element 111 is also coupled to a respective data line BL0-BLm (e.g., bit line). Each bit line BL0-BLm is coupled to its respective column of memory cells.

The word lines WL0-WLn are coupled to one or more access (e.g., row) decoders (not shown) that are used to selectively access the word lines. The bit lines BL0-BLm are coupled to the sense circuitry, through a decoder hierarchy, that senses either a voltage or a current in order to determine the programmed state of the respective memory cells that have been accessed by the word line.

One problem with PCM is the thermal transfer that can occur when an adjacent memory cell is programmed. This is typically referred to in the art as thermal disturb. The heat resulting from the programming process of one memory cell can transfer to the adjacent memory cell causing it to either be programmed or change the resistance of the already programmed cell, thus changing the valued of the stored data.

For the reasons stated above and for other reasons that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for reducing the effects of thermal disturb in a phase change memory device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a portion of a typical prior art PCM array.

FIG. 2 shows a cross-sectional view of one embodiment of PCM cells.

FIG. 3 shows an onset of disturb versus a distance between memory cells along a bit line.

FIG. 4 shows schematic diagram of a portion of a PCM array.

FIG. 5 shows flowchart of one embodiment of a method for programming a PCM array.

FIG. 6 shows a block diagram of one embodiment of a system that can incorporate a memory device in accordance with the PCM array of FIG. 4.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings that form a part hereof and in which is shown, by way of illustration, specific embodiments. In the drawings, like numerals describe substantially similar components throughout the several views. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present disclosure. The following detailed description is, therefore, not to be taken in a limiting sense.

FIG. 2 illustrates a cross-sectional view, in a bit line direction, of one embodiment of PCM cells and their supporting structure. The cross-sectional view shows a top layer 201 that can be a metal layer acting as a bit line or coupled to a bit line. The top layer 201 is formed on a capping layer 202 that is formed over a resistance variable material (e.g., phase change material). The resistance variable (e.g., phase change) material can be a chalcogenide material (e.g., GeSbTe) that can be referred to as GST but alternate embodiments can include another resistance variable material.

The cross-sectional view shows three memory cell areas 250-252 that can be formed within the GST layer 203. The memory cell areas 250-252 are above the memory cell heaters 210-212 such that each memory cell 250-252 has a different heater 210-212. The heaters 210-212 have a SiN material on either side.

The heaters 210-212 are formed over plugs 205-207 that each act as, or are coupled to, a word line. The plugs 205-207 are separated by a dielectric material 260, 261. One of the plugs 206 is labeled as an “A” plug and an adjacent plug 207 is labeled as a “B” plug. The use of these labels will become evident subsequently with reference to FIG. 4.

It can be seen that the heaters 210-212 are formed as an “L” shape. A SiN material 220-222 is formed on the one side of each “L” heater. An insulator material 216 (e.g., SiO) is formed between the two SiN layers 220, 221. The memory cell 251 associated with the center-most heater 211 is considered to be in a Front-to-Front (FTF) relationship with respect to the left-most memory cell 252.

The heater 210 associated with the right-most memory cell 250 has its “L” shape pointed away from the center heater 211 (e.g., the “L” shaped heaters are back-to-back). These two heaters 210, 211 are separated by SiN 217. The memory cell 250 associated with the right-most heater 210 is considered to be in a Back-to-Back (BTB) relationship with respect to the center-most memory cell 251.

The left-most memory cell 252 is also considered to be in a BTB relationship with respect to an adjacent memory cell to the left (not shown) for the same reasoning. The left-most heater 212 also has SiN 215 between it and the adjacent heater (not shown).

Thus, it can be seen from FIG. 2 that BTB memory cells 250, 251 share a SiN region along the bit line direction. FTF memory cells 251, 252 share a SiO insulator along the bit line direction.

In operation, the memory cells 250-252 are programmed by providing a current through the heaters 210-212 causing an increase in temperature of the memory cell area in the GST layer 203 above the respective heater 210-212. When the temperature is high enough to melt the area of the GST layer 203 above the heater 210-212, an amorphous dome 250-252 is formed in the GST layer 203 that increases the resistance of that area, thus programming that particular memory cell.

Since an amorphous material is metastable, the memory cell 250-252 can be erased by increasing the temperature of the memory cell to a crystallization temperature that is typically lower than the temperature used to program the memory cell. The crystallization temperature causes the amorphous dome to crystallize back to the crystal state, thus erasing the memory cell.

The time required to program and/or erase the memory cells depends on the temperatures used. A small increase of the temperature translates into faster crystallization according to the exponential relationship between time and temperature, i.e., the crystallization Arrhenius Law. After multiple write/erase operations of a neighbor memory cell, a previously programmed bit can be potentially stressed for many seconds (e.g., number of disturb operations of neighbor cell x time of the single pulse). This corresponds to a temperature on the disturbed memory cell of 170°-250° C. that is lower than a programming temperature but still potentially disturbing for that length of time.

It can be seen that heat from one heater might be conducted along the GST material 203 to an adjacent memory cell, thus causing thermal disturb in the adjacent memory cells. For example, referring to FIG. 2, if the right-most memory cell 250 is already programmed, programming of the center-most memory cell 251 can cause the temperature of the right-most memory cell 250 to also increase. Over a number of programming operations, this can cause thermal disturb of programmed memory cells.

In one specific embodiment, memory cells typically suffer thermal disturb in the bit line direction. Since the memory cells are insulated from adjacent memory cells in the word line direction, thermal disturb is not normally a problem in the word line direction. In one embodiment, the thermal disturb is worse in the bit line direction since the GST is continuous (i.e., not cut). In the word line direction, the GST is cut between memory cells so that the temperature is more limited.

In one specific embodiment, the thermal disturb in the bit line direction is not symmetrical. Referring to FIG. 2, it can be seen that the heater 211 for the center memory cell 251 is spaced from the left-most heater 212 by a distance of L_(x) and the center memory cell 251 is spaced from the left-most memory cell 252 by a distance of L_(y). Meanwhile, the heater 211 for the center memory cell 251 is spaced from the right-most heater 210 by a distance of L_(x′) and the center memory cell 251 is spaced from the right-most memory cell 250 by a distance of L_(y′). As shown in FIG. 3, the BTB memory cell pairs are more susceptible to thermal disturb than the FTF memory cell pairs.

FIG. 3 illustrates the current number of cycles performed on a neighbor memory cell that could result in data loss in an observed memory cell (1 μA is the data loss criterion) as a function of the spacing between the memory cells for the specific embodiments of FIG. 2. The two curves refer to FTF or BTB memory cells. The x-axis of the graph is the distance L_(y) in nanometers. The y-axis of the graph is the onset of thermal disturb at 1 μA as a number of program/erase cycles.

By picking a point 300 on the graph at L_(y)=109 nm, it can be seen that the number of program/erase cycles that can cause disturb in FTF memory cell pairs is substantially much more than the number of program/erase cycles that can cause thermal disturb in BTB memory cell pairs. Thus, a programming method can reduce the occurrence of thermal disturb by, for example, simultaneously programming pairs of adjacent data bits that can cause thermal disturb i.e., in the embodiment of FIG. 2 BTB pairs. This is further illustrated in the schematic of FIG. 4.

FIG. 4 illustrates a schematic diagram of a portion of a resistive memory array (e.g., PCM array) 400. The PCM array 400 includes a number of memory cells 401, each having a select device 410 coupled to a resistive storage element 411. The memory cells 401 can include heater elements, conductive elements, and resistive storage elements as described previously. The resistive storage elements 411 can include a resistance variable material (e.g., phase change material).

The select devices 410 can include field effect transistors (FETs) (e.g., MOSFETs), bipolar junction transistors (BJTs), BJTs operated as diodes, or diodes. Although the select devices 410 are shown in FIG. 4 as two terminal select devices, alternate embodiments can use other select devices 410 with a different number of terminals.

Referring to FIG. 4, the select device 410 is shown as a two terminal device where the cathode terminal of each select device 410 is coupled to one of a number of access lines (e.g., word lines WL0-WL4). Each word line WL0-WL4 is coupled in such a fashion to its respective row of memory cells. The word lines WL0-WL4 are coupled to sense and programming circuitry. Only five word lines are shown in FIG. 4 for purposes of clarity and brevity. One skilled in the art will realize that a memory device can have a large number of word lines in an array.

The anode terminal of each select device 410 is coupled to a respective resistive storage element 411 of a respective memory cell. Each resistive storage element 411 is also coupled to a respective data line (e.g., bit lines BL0-BLm). Each bit line BL0-BLm is coupled to its respective column of memory cells. The bit lines BL0-BLm are coupled to access decoders. Only three bit lines are shown in FIG. 4 for purposes of clarity and brevity. One skilled in the art will realize that a memory device can have a large number of bit lines in an array. The select devices 410 are enabled/disabled (e.g., turned on/off) in order to select/deselect its respective memory cell 401.

One word line 450 coupled to a memory cell 402 is labeled A while its adjacent word line 451 coupled to a second memory cell 401 is labeled B. It can be seen that the first memory cell 402 shares a BTB relationship with the second memory cell 401. Referring to the cross-sectional view of FIG. 2, it can be seen that the memory cell 251 associated with the heater coupled to plug A 206 shares a common SiN material 217 with the memory cell 250 associated with the heater coupled to plug B 207.

By programming data to the BTB memory cells 401, 402 simultaneously, thermal disturb can be reduced. Thus, while programming memory cells in the bit line direction, when a memory cell is being programmed, it is programmed along with an adjacent memory cell with which it shares a BTB relationship. In order to accomplish this, the word lines are treated like typical bit lines of the prior art in that the word lines of the array of FIG. 4 are coupled to sense and programming circuitry. Similarly, the bit lines of the array of FIG. 4 are treated like typical word lines of the prior art in that the bit lines of the array of FIG. 4 are coupled to selecting access decoders.

FIG. 5 illustrates a flowchart of one embodiment of a method for programming the array of FIG. 4. This method can reduce the thermal disturb typically experienced by adjacent memory cells along a given axis (e.g., in a bit line direction).

The method initially builds a word to be programmed by combining data bits into multiple bit pairs that can be programmed into adjacent memory cells 501. In one embodiment, the bits are combined into bit pairs such that two memory cells that share a BTB relationship are programmed during the same programming operation so that they are programmed simultaneously. In one embodiment, each adjacent BTB memory cell pair shares the same SiN material.

The building of the logical word for programming can also be referred to as remapping the data from one location to another. The data that is remapped to different locations for the purpose of programming as a pair of data bits is tracked so that when the data is read out of the memory, it can be mapped back to its original location in the data word.

For example, if a memory device receives, from an external controller, a 32-bit logical data word to be programmed, the logical data word might include a data bit in one location of the word that is to be programmed while the adjacent bit location is not to be programmed. The data remapping would replace the data bit that originally was not to be programmed with a data bit to be programmed. The resulting pair could also be moved within the logical word such that one bit is programmed to a first memory cell and the other to a second memory cell having a BTB relationship with the first memory cell and that shares a SiN material with the first memory cell.

The selected bit line to be programmed is biased with a relatively high programming voltage 503. In one embodiment, the high programming voltage is a series of incrementally increasing programming pulses. Since the row decoders are coupled to the bit lines, the selecting row decoder can provide the relatively high programming voltage. In an alternate embodiment, the programming voltage can be a negative voltage such that the programming pulses are a series of incrementally more negative programming pulses.

The word lines that are coupled to the memory cells being programmed along the selected bit line are also biased with an appropriate voltage for programming the memory cells 505. In one embodiment, this is a relatively low voltage (e.g., 0V) as compared to the programming voltage.

FIG. 6 illustrates a functional block diagram of a memory device 600 that can comprise a PCM array architecture such as illustrated in FIG. 4 and can be programmed using the programming methods disclosed herein. The memory device 600 is coupled to an external controller 610. The external controller 610 may be a microprocessor or some other type of controller. The memory device 600 and the external controller 610 form part of a system 620.

The memory device 600 includes an array 630 of memory cells (e.g., PCM memory cells). The memory array 630 is arranged in banks of word line rows and bit line columns.

Address buffer circuitry 640 is provided to latch address signals provided through I/O circuitry 660. Address signals are received and decoded by a row decoder 644 and a column decoder 646 to access the memory array 630.

The memory device 600 reads data in the memory array 630 by sensing resistance changes in the memory array columns using sense circuitry 650. The sense circuitry 650, in one embodiment, is coupled to read and latch a row of data from the memory array 630. The sense circuitry 650, as previously described, includes the sense circuitry as well as other circuits for performing a program verify operation. Data are input and output through the I/O circuitry 660 for bidirectional data communication as well as the address communication over a plurality of data connections 662 with the controller 610.

An internal controller (e.g., control circuitry and firmware) 670 decodes signals provided on a control interface 672 from the external controller 610. These signals are used to control the operations on the memory array 630, including data read, data write (program), and erase operations. The internal controller 670 may be a state machine, a sequencer, or some other type of controller to generate the memory control signals. In one embodiment, the internal controller 670 is configured to control execution of the programming embodiments of the present disclosure. In one embodiment, the internal controller is responsible for tracking the remapping of the data into data bit pairs such that the data can be read properly during a read operation.

The memory device illustrated in FIG. 6 has been simplified to facilitate a basic understanding of the features of the memory. A more detailed understanding of internal circuitry and functions of flash memories are known to those skilled in the art.

CONCLUSION

In summary, one or more embodiments of a resistive memory array that is programmed such that adjacent pairs of memory cells along a bit line in a BTB relationship are programmed together. By programming pairs of adjacent memory cells in a BTB relationship along a bit line, the thermal disturb of memory cells during programming can be reduced. Other embodiments have also been discussed.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the invention. 

What is claimed is:
 1. A method for programming a data word to an array of memory cells, wherein the array of memory cells comprise a first memory cell, a second memory cell that is adjacent to the first memory cell in a first direction along an axis, and a third memory cell that is adjacent to the first memory cell in a second direction along the axis, wherein the first direction is opposite to the second direction and the first memory cell is more thermally insulated from the second memory cell than the first memory cell is thermally insulated from the third memory cell, the method comprising programming the data word to the first and the third memory cells.
 2. The method of claim 1 wherein the array of memory cells further comprises a fourth memory cell that is adjacent to the second memory in the first direction along the axis, and wherein programming the data word to the first and the third memory cell comprises programming a first data word to the first and the third memory cell, the method further comprising programming a second data word to the second and the fourth memory cells.
 3. The method of claim 1 wherein programming the data word comprises programming a pair of data bits to adjacent, back-to-back memory cells along a data line of the array of memory cells.
 4. The method of claim 1 and further comprising building the data word comprising a plurality of pairs of data bits wherein each pair of data bits is to be programmed to adjacent memory cells that share a SiN material between the adjacent memory cells.
 5. The method of claim 4 wherein at least one data bit of the pair of data bits is remapped data from another location in a received data word.
 6. The method of claim 5 and further comprising remapping locations of the data word that are not to be programmed with data that are to be programmed.
 7. A method for programming data to an array of memory cells, wherein the array of memory cells comprise a first memory cell, a second memory cell that is adjacent to the first memory cell in a first direction along a first axis, and a third memory cell that is adjacent to the first memory cell in a second direction along a second axis, wherein the first and third memory cells are back-to-back memory cells that share a same chalcogenide material area that is not cut, the method comprising programming the data to the first and the third memory cells.
 8. The method of claim 7 wherein the chalcogenide material is GeSbTe.
 9. The method of claim 7 chalcogenide material is formed in a bit line direction.
 10. An apparatus comprising: an array of memory cells comprising: a first memory cell; a second memory cell that is adjacent to the first memory cell in a first direction along an axis; and a third memory cell that is adjacent to the first memory cell in a second direction along the axis, wherein the first direction is opposite of the second direction and the first memory cell is more thermally insulated from the second memory cell than the first memory cell is thermally insulated from the third memory cell; and circuitry configured to program a logical word into the first and the third memory cells.
 11. The apparatus of claim 10, wherein the first memory cell, the second memory cell, and the third memory cell comprise a shared layer of resistance variable material, and wherein the first memory cell comprises a first memory cell heater, the second memory cell comprises a second memory cell heater, and the third memory cell comprise a third memory cell heater.
 12. The apparatus of claim 11, wherein a first material is between the first and second memory cell heaters and a second material is between the first and third memory cell heaters.
 13. The apparatus of claim 12, wherein the first material comprises silicon oxide and the second material comprises silicon nitride.
 14. The apparatus of claim 12, wherein the layer of resistance variable material is over the first material, the second material, the first memory cell heater, the second memory cell heater, and the third memory cell heater.
 15. The apparatus of claim 13, further comprising metal over the layer of resistance variable material along the axis.
 16. The apparatus of claim 14, further comprising a capping layer between the layer of resistance variable material and the metal.
 17. The apparatus of claim 12, further comprising a decoder, wherein the metal comprises a metal line, and wherein the first, second and third memory cells are coupled to the decoder via the metal line.
 18. The apparatus of claim 12, wherein the first heater is separated from the third heater by only the first material.
 19. The apparatus of claim 10, wherein the first heater is separated from the second heater by a first spacer formed of the first material, the second material and a second spacer formed of the first material.
 20. The apparatus of claim 10, wherein the array further comprises a fourth memory cell that is adjacent to the second memory in the first direction along the axis, wherein the codeword comprises a first codeword and wherein the circuitry is further configured to program a second codeword to the second and the fourth memory cells.
 21. A system comprising: a controller configured to control the system; and a resistive memory device coupled to the controller, the memory device comprising: an array of resistive memory cells organized in rows and columns wherein each column of memory cells is coupled to a data line and each row of memory cells is coupled to an access line; sense circuitry coupled to the access lines and configured to read data stored in the array of resistive memory cells; row decoders coupled to the data lines and configured to bias a selected data line with programming pulses during a programming operation; and control circuitry configured to build a data word from remapped data bits from a received data word such that pairs of data bits are mapped to adjacent locations in the built data word, the control circuitry further configured to program the built data word to memory cells coupled to the selected data line such that, during a same program operation, the pairs of adjacent memory cells along the selected data line are programmed with the pairs of data bits wherein the pairs of adjacent memory cells comprise memory cells that have a back-to-back relationship.
 22. The system of claim 21 wherein each memory cell comprises a resistive storage element and a select device.
 23. The system of claim 22 wherein the select device comprises one of: a MOSFET, a bipolar junction transistor, or a diode.
 24. The system of claim 22 wherein the select device is one of: a two terminal device or a three terminal device.
 25. The system of claim 21 wherein the memory cells having the back-to-back relationship share a SiN material. 